The energy level spectrum of gap states in P-doped a-Si: H has been determined using photo-isothermal capacitance transient spectroscopy (ICTS) with sub-bandgap excitation. Photocapadtance transients were observed in the range from the bandgap energy down to 0·5 eV. It has been deduced that two gap-state features exist in the mobility gap; one is 0·5−0·6 eV below the mobility edge (Ec) of the conduction band, and the other 1·0−1·2 eV belowEc. These results are consistent with our previously reported results on ICTS as well as with defect PL, which were explainedin terms of ‘isolated’ and ‘charge-coupled’ doubly occupied dangling bonds (D−,*D−).